Normal hemostasis is the result of a delicate balance between the processes of clot formation (blood coagulation) and clot dissolution (fibrinolysis). The complex interactions between blood cells, specific plasma proteins and the vascular surface, maintain the fluidity of blood unless injury occurs. Damage to the endothelial barrier lining the vascular wall exposes underlying tissue to these blood components. This in turn triggers a series of biochemical reactions altering the hemostatic balance in favor of blood coagulation which can either result in the desired formation of a hemostatic plug stemming the loss of blood or the undesirable formation of an occlusive intravascular thrombus resulting in reduced or complete lack of blood flow to the affected organ.
The blood coagulation response is the culmination of a series of amplified reactions in which several specific zymogens of serine proteases in plasma are activated by limited proteolysis. This series of reactions results in the formation of an insoluble matrix composed of fibrin and cellular components which is required for the stabilization of the primary hemostatic plug or thrombus.
The initiation and propagation of the proteolytic activation reactions occurs through a series of amplified pathways which are localized to membranous surfaces at the site of vascular injury (Mann, K. G., Nesheim, M. E., Church, W. R., Haley, P. and Krishnaswamy, S. (1990) Blood 76: 1-16. and Lawson, J. H., Kalafatis, M., Stram, S., and Mann, K. G. (1994) J. Biol. Chem. 269: 23357-23366).
Initiation of the blood coagulation response to vascular injury follows the formation of a catalytic complex composed of serine protease factor VIIa and the non-enzymatic co-factor, tissue factor (TF)(Rappaport, S. T. and Rao, L. V. M. (1992) Arteriosclerosis and Thrombosis 12: 1112-1121). This response appears to be exclusively regulated by the exposure of subendothelial TF to trace circulating levels of factor VIIa and its zymogen factor VII, following a focal breakdown in vascular integrity. Autoactivation results in an increase in the number of factor VIIa/TF complexes which are responsible for the formation of the serine protease factor Xa. It is believed that in addition to the factor VIIa/TF complex, the small amount of factor Xa which is formed primes the coagulation response through the proteolytic modification of factor IX to factor IX.sub.alpha which in turn is converted to the active serine protease factor IX.sub.ab by the factor VIIa/TF complex (Mann, K. G., Krishnaswamy, S. and Lawson, J. H. (1992) Sem. Hematology 29: 213-226.). It is factor IXa.sub.b in complex with activated factor VIIIa, which appears to be responsible for the production of significant quantities of factor Xa which subsequently catalyzes the penultimate step in the blood coagulation cascade; the formation of the serine protease thrombin.
Factor Xa catalyzes the formation of thrombin following the assembly of the prothrombinase complex which is composed of factor Xa, the non-enzymatic co-factor Va and the substrate prothrombin (factor II) assembled in most cases, on the surface of activated platelets which are adhered at the site of injury (Fuster, V., Badimon, L., Badimon, J. J. and Chesebro, J. H. (1992) New Engl. J. Med. 326: 310-318). In the arterial vasculature, the resulting amplified "burst" of thrombin generation catalyzed by prothrombinase results locally high levels of this protease which is responsible for the formation of fibrin and the further recruitment of additional platelets as well as the covalent stabilization of the clot through the activation of the transglutaminase zymogen factor XIII. In addition, the coagulation response is further propagated through the thrombin-mediated proteolytic feedback activation of the non-enzymatic co-factors V and VIII resulting in more prothrombinase formation and subsequent thrombin generation (Hemker, H. C. and Kessels, H. (1991) Haemostasis 21: 189-196).
Substances which interfere in the process of blood coagulation (anticoagulants) have been demonstrated to be important therapeutic agents in the treatment and prevention of thrombotic disorders (Kessler, C. M. (1991) Chest 99: 97S-112S and Cairns, J. A., Hirsh, J., Lewis, H. D., Resnekov, L., and Theroux, P. (1992) Chest 102: 456S-481S). The currently approved clinical anticoagulants have been associated with a number of adverse effects owing to the relatively non-specific nature of their effect on the blood coagulation cascade (Levine, M. N., Hirsh, J., Landefeld, S., and Raskob, G. (1992) Chest 102: 352S-363S). This has stimulated the search for more effective anticoagulant agents which can more effectively control the activity of the coagulation cascade by selectively interfering with specific reactions in this process which may have a positive effect in reducing the complications of anticoagulant therapy (Weitz, J., and Hirsh, J. (1993) J. Lab. Clin. Med. 122: 364-373). In another aspect, this search has focused on normal human proteins which serve as endogenous anticoagulants in controlling the activity of the blood coagulation cascade. In addition, various hematophageous organisms have been investigated because of their ability to effectively anticoagulate the blood meal during and following feeding on their hosts suggesting that they have evolved effective anticoagulant strategies which may be useful as therapeutic agents.
A plasma protein, Lipoprotein-Associated Coagulation Inhibitor (LACI) or recently termed Tissue Factor Pathway Inhibitor (TFPI), containing three consecutive Kunitz domains has been reported to inhibit the enzyme activity of factor Xa directly and, in a factor Xa-dependent manner, inhibit the enzyme activity the factor VIIa-tissue factor complex. Salvensen, G., and Pizzo, S. V., "Proteinase Inhibitors: a-Macroglobulines, Serpins, and Kunis", "Hemostasis and Thrombosis, Third Edition, pp. 251-253, J. B. Lippincott Company (Edit. R. W. Colman et al. 1994). A cDNA sequence encoding TFPI has been reported, and the cloned protein was reported to have a molecular weight of 31,950 daltons and contain 276 amino acids. Broze, G. J. and Girad, T. J., U.S. Pat. No. 5,106,833, col. 1, (1992). Various recombinant proteins derived from TFPI have been reported. Girad, T. J. and Broze, G. J., EP 439,442 (1991); Rasmussen, J. S. and Nordfand, O. J., WO 91/02753 (1991); and Broze, G. J. and Girad, T. J., U.S. Pat. No. 5,106,833, col. 1, (1992).
Antistasin, a protein comprised of 119 amino acids and found in the salivary gland of the Mexican leech, Haementeria officinalis, has been reported to inhibit the enzyme activity of factor Xa. Tuszynski et al. (1987) J. Biol. Chem, 262:9718; Nutt, et al. (1988) J. Biol. Chem, 263:10162. A 6,000 daltons recombinant protein containing 58 amino acids with a high degree homology to antistasin's amino-terminus amino acids 1 through 58 has been reported to inhibit the enzyme activity of factor Xa. Tung, J. et al., EP 454,372 (1991); Tung, J. et al., U.S. Pat. No. 5,189,019 (1993).
Tick Anticoagulant Protein (TAP), a protein comprised of 60 amino acids and isolated from the soft tick, Ornithodoros moubata, has been reported to inhibit the enzyme activity of factor Xa but not factor VIIa. Waxman, L. et al. (1990) Science, 248:593. TAP made by recombinant methods has been reported. Vlausk, G. P. et al., EP 419,099 (1991) and Vlausk, G. P. et al., U.S. Pat. No. 5,239,058 (1993).
The dog hookworm, Ancylostoma caninum, which can also infect humans, has been reported to contain a potent anticoagulant substance. A. caninum was reported to contain a substance which inhibited coagulation of blood in vitro. Loeb, L. and Smith, A. J. (1904) Proc. Pathol. Soc. Philadelphia, 7:173-178. Extracts of A. caninum were reported to prolong prothrombin time and partial thromboplastin time in human plasma with the anticoagulant effect being reported attributable to inhibition of factor Xa but not thrombin. Spellman, Jr., J. J. and Nossel, H. L. (1971) Am. J. Physiol., 220:922-927. More recently, soluble protein extracts of A. caninum were reported to prolong prothrombin time and partial thromboplastin time in human plasma in vitro. The anticoagulant effect was reported to be attributable to inhibition of human factor Xa but not thrombin. Cappello, M, et al. (1993) J. Infect. Diseases, 167:1474-1477.